The Eyes Have It

By Ian Kendall - Tuesday, March 5, 2019

Ok, so why do we need a biology lesson? It’s all about the circle of focus – this is something I’ve written and lectured about many times, and I make no apologies for doing it again, because I think it’s that important.

So, consider the eye. It’s a ball (almost) around the size of a ping pong ball. On the inside coating the inner wall is something called the retina – think of this as the film in a camera. The retina is made up of two types of cells – rods and cones – and they do very different things. Rods detect light and dark, work at low light levels and not much else. They are the 1950s TV sets of the eye. Cones, on the other hand, react well to good light, can detect colour and are very responsive. They are the 4K TVs in this tortured analogy. But why is this important?

I’m glad you asked. If you draw an imaginary line to the back of the eye through the centre of the lens, you come to a small indentation in the retina. It’s called the fovea and it contains the largest concentration of cones in your eye, and very, very few rods (if any). The rest of the retina is mostly rods, with a few cones for peripheral colour vision. But why is this important?

If you are looking directly at something, the light goes through the lens, and hits the retina where you have excellent detailed colour vision. This, on an evolutionary track, is very beneficial. This is also why you cannot see a star if you look directly at it; there are no rods in the fovea to pick up the low light of the star, and you have to look slightly off to the side before it comes into view (try this tonight and you’ll understand what’s happening).

What does this have to do with magic? If you are performing close up magic, there’s a good chance that you will be standing within a metre of your audience (two, at most). At this range, someone has a 45cm (18”) circle of focus – anything outside this lies in peripheral vision, and cannot be seen with such clarity. In close up we can use this by performing sleights at waist level while looking the spectator in the eye, which gives us a bit more cover. On stage, however, you are always going to be more than two metres from your audience, and so all of you is now in this circle of focus.

This means we have to be more thoughtful when it comes to sleights on stage; a pass, for example, might be easy to cover with misdirection up close, but the action becomes a lot more visible on stage. There are a couple of thing we can do, especially for the pass, but I’ll be covering stand up sleights in a later post. In the meantime, think of the sleights you use in your routines. Set up a camera as far away from you as you can fit (so you have a full body shot at least), and then run through your moves. Watch the video back with a critical eye and write down which moves suddenly become glaringly visible. We’ll deal with them in a short while.

Reader comments:

Gustav

This is a really interesting way of thinking about vision. I have spent the last 15 years of more studying misdirection using eye tracking techniques, and have never thought about it in this way. One thing to keep in mind is that people can often miss seeing things even if they are looking at them. In the misdirection tricks that we have been using to study attentional misdirection, we find very little relationship between where people look and what they see. You can find a review of some of this work here. https://www.magicresearchlab.com/misdirection

For this interested in the psychology of magic, I have just published a book on the Science of Magic, Experiencing the Impossible, which discusses the visual processes involved in misdirection in much detail (see chapter 4 - the grand illusion).

https://mitpress.mit.edu/books/experiencing-impossible

The book also looks at other forms of misdirection such as memory illusions, perceptual illusions, and the illusion of free will.